The 10 Most Important Dinosaur Facts

The first dinosaurs evolved during the middle to late Triassic period, about 230 million years ago, in the part of the supercontinent of Pangea that now corresponds to South America. Before then, the dominant land reptiles were archosaurs ("ruling lizards"), therapsids ("mammal-like reptiles") and pelycosaurs (typified by Dimetrodon), and for 20 million or so years after dinosaurs evolved the most fearsome reptiles on earth were prehistoric crocodiles. It was only at the beginning of the Jurassic period, 200 million years ago, that dinosaurs truly began their rise to dominance.

With our 100-year-max life spans, human beings aren't well adapted to understanding "deep time," as geologists call it. To put things in perspective: modern humans have only existed for a few hundred thousand years, and human civilization only got started about 10,000 years ago, mere blinks of the eye by Jurassic time scales. Everyone talks about how dramatically (and irrevocably) the dinosaurs went extinct, but judging by the whopping 165 million years they managed to survive, they may have been the most successful vertebrate animals ever to colonize the earth!

Not every paleontologist is convinced, and there are some alternate (albeit not widely accepted) theories. But the bulk of the evidence points to modern birds having evolved from small, feathered, theropod dinosaurs during the late Jurassic and Cretaceous periods. Bear in mind, though, that this evolutionary process may have happened more than once, and that there were definitely some "dead ends" along the way (witness the tiny, feathered, four-winged Microraptor, which has left no living descendants). In fact, if you look at the tree of life cladistically--that is, according to shared characteristics and evolutionary relationships--it's completely appropriate to refer to modern birds as dinosaurs.

 Modern reptiles like turtles and crocodiles are cold-blooded, or "ectothermic," meaning they need to rely on the environment to maintain their internal body temperatures--while modern mammals and birds are warm-blooded, or "endothermic," possessing active, heat-producing metabolisms that maintain a constant internal body temperature, no matter the external conditions. There's a solid case to be made that at least some meat-eating dinosaurs--and even a few ornithopods--must have been endothermic, since it's hard to imagine an active lifestyle being fueled by a cold-blooded metabolism. (On the other hand, it's unlikely that giant dinosaurs like Argentinosaurus were warm-blooded, since they would have cooked themselves from the inside out.)

 Fierce carnivores like Tyrannosaurus Rex and Giganotosaurus get all the press, but it's a fact of nature that the meat-eating "apex predators" of any given ecosystem are tiny in number compared to the plant-eating animals on which they feed (and which themselves rely on the vast amounts of vegetation needed to sustain such large populations). By analogy with modern ecosystems in Africa and Asia, herbivorous hadrosaurs, ornithopods and (to a lesser extent) sauropods probably roamed the world's continents in vast herds, hunted by sparser packs of large, small and medium-sized theropods.

 It's true, some plant-eating dinosaurs (like Stegosaurus) had brains so tiny compared to the rest of their bodies that they must have been only a little bit smarter than giant ferns. But meat-eating dinosaurs large and small, ranging from Troodon to T. Rex, possessed more respectable amounts of grey matter compared to their body size, since they required better-than-average sight, smell, agility and coordination to reliably hunt down prey.

 Many people mistakenly believe that mammals "succeeded" the dinosaurs 65 million years ago, appearing everywhere, all at once, to occupy the ecological niches rendered vacant by the K/T Extinction Event. The fact is, though, that early mammals lived alongside sauropods, hadrosaurs, and tyrannosaurs (usually high up in trees, out of harm's way) for most of the Mesozoic Era, and in fact they evolved at around the same time (the late Triassic period, from a population of therapsid reptiles).  Most of these early furballs were about the size of mice and shrews, but a few (like the dinosaur-eating Repenomamus) grew to respectable sizes of 50 pounds or so.

 It may seem like nitpicking, but the word "dinosaur" applies only to land-dwelling reptiles possessing a specific hip and leg structure, among other anatomical characteristics; here's an article explaining the scientific definition of a dinosaur. As large and impressive as some genera (such as Quetzalcoatlus and Liopleurodon) were, flying pterosaurs and swimming plesiosaurs, ichthyosaurs and mosasaurs weren't dinosaurs at all--and some of them weren't even all that closely related to dinosaurs, save for the fact that they're also classified as reptiles.